The present invention relates to a constant volume valve for a combustion-powered tool, such as a power framing tool. More specifically, it relates to a constant volume valve assembly that measures a volume of a fluid before allowing it to flow into a combustion chamber.
This invention also relates to a pneumatically powered, combustion-powered, or other rapidly acting, fastener-driving tool of a type utilizing collated fasteners. Typically, as exemplified in Nikolich U.S. Pat. Re. 32,452, Nikolich U.S. Pat. No. 4,522,162; Nikolich U.S. Pat. No. 4,483,474; Nikolich U.S. Pat. No. 4,403,722 and Wagdy U.S. Pat. No. 4,483,473, which are herein incorporated by reference, a combustion-powered, fastener-driving tool comprises a combustion chamber, which is defined by a cylinder body and by a valve sleeve arranged for opening and closing the combustion chamber. Generally, similar combustion-powered, nail- and staple-driving tools are available commercially from ITW-Paslode (a unit of Illinois Tool Works Inc.) of Vernon Hills, Ill., under its IMPULSE trademark.
In such a tool, it is beneficial to apply a constant force during the driving stroke to each fastener as it is driven into the workpiece. Measurement of the amount of fuel to the combustion-powered tool, or the amount of compressed gas to a pneumatically powered tool, helps provide a constant force. A combustion powered fastening tool is described in U.S. Pat. No. 4,721,240 to Cotta that measures fuel by opening a valve for a length of time defined by movement of a cam. Fuel passes through a fuel valve to a combustion chamber conduit, the amount of which is equal to the volume that passes through a needle valve during the time the fuel valve is open. Measurement of the flow of a fluid by time allows the amount of fluid supplied to the tool to vary as flow rates of the fluid change. As a fuel cylinder is emptied, the flow rate of the fluid changes as the cylinder pressure drops. Similarly, pressure or flow variations in a common supply of pneumatic fluid will also result in differences in the amount of power supplied on each charge of the cylinder.
Control of fuel into a combustion chamber by valve assemblies is shown in U.S. Pat. Nos. 655,996 and 1,293,858. Both references disclose a pressurized fluid inlet valve and fluid outlet valve that bracket a machine-supply passage. High-pressure fluid is fed to a machine to supply power via the inlet valve, and is discharged through the outlet valve when it returns from the machine following expulsion of its power. Neither reference teaches the use of such a system to supply a constant measurement of fluid. Further, following combustion of a fuel or expansion of a high-pressure fluid, the fluid is no longer useful to supply power to a tool and measurement at that point is ineffective.
U.S. Pat. No. 4,913,331 to Utsumi describes an apparatus that drives a piston with an internal combustion engine that utilizes a measuring chamber to dispense a constant volume of fuel. A fuel piston containing the measuring chamber is reciprocally moveable within a fuel cylinder. The fuel inlet channel and the fuel outlet channel are positioned such that the measuring chamber is filled and emptied by movement of the piston between the inlet and outlet channels. Seals are located on either side of the chamber between the fuel piston and the cylinder, preventing leakage of fuel from the pressurized fuel supply to the combustion chamber. Steady movement of the piston would cause rapid wear on these seals, since they are constantly in contact with the cylinder surface.
One operational drawback of conventional combustion-powered tools, is that when operated at relatively low temperatures, such as below 32° F., the pressure of the pressurized fuel falls, causing a greater pressure differential between the atmosphere and the fuel. At this lower pressure, the fuel does not dissipate as rapidly through the appropriate passageways and into the combustion chamber. This condition causes a delay in the combustion, which interferes with the operational efficiency of the tool.
Another operational drawback of conventional combustion-powered tools, is that when operated at relatively higher elevations or altitudes, there is less air for combustion. As a result, when used at such higher elevations, conventional combustion-powered tools with constant volume fuel metering valves can have overly rich fuel/air mixtures in their combustion chambers, which can lead to fouling of the ignition system as well as other operational difficulties. As such, there is a need for a combustion-powered tool with a fuel metering valve which has the capability of adjusting the amount of fuel in the combustible fuel/air mixture.
It is, therefore, an object of this invention to provide an improved constant volume measurement of a fluid to an apparatus, such as a combustion-powered tool, to produce a constant driving force.
It is yet another object of this invention to provide an improved constant volume measurement of fluid in a compact space.
It is still another object of this invention to provide an improved constant volume valve assembly, whose seals are not constantly wearing against a sealing surface.
It is a further object of the present invention to provide an improved constant volume valve assembly that facilitates the movement of fuel even when fuel pressure drops, such as when the tool is exposed to low temperatures.
It is a still further object of the present invention to provide an improved constant volume valve assembly that provides the capability of adjusting the fuel mixture, such as when the tool is operated at relatively high elevations.